Chair base



FIG. 1 is a top perspective view of the chair base;

FIG. 2 is an underneath perspective view of the chair base of FIG. 1;

FIG. 3 is a top view of the chair base of FIG. 1;

FIG. 4 is a front view of the chair base of FIG. 1;

FIG. 5 is a bottom view of the chair base of FIG. 1;

FIG. 6 is right side view of the chair base of FIG. 1.

FIG. 7 is left side view of the chair base of FIG. 1; and,

FIG. 8 is a rear view of the chair base of FIG. 1.

The portions shown in broken line are not part of the design as claimed. 

The ornamental design for a chair base, as shown and described. 